In an automobile, in order to increase travelling performance, maneuverability, and fuel consumption, a vehicle body is required to be light in weight. Therefore, materials having low specific gravity, such as aluminum alloy, are beginning to be applied to various automobile body parts. For example, an automobile panel structure, such as a hood (bonnet), a door, or a trunk, is a hollow structure including an outer panel (outer plate) and an inner panel (inner plate). The use of aluminum alloy materials as materials of these panels is being considered.
Although, in this case, the materials of the outer panel and the inner panel may be changed to aluminum alloy plates, they are sometimes formed into a different materials panel structure in which materials having characteristics that are in accordance with the required characteristics of the panels are used in combination. More specifically, the outer panel that is required to have, for example, designability, to be light in weight, and to have collision energy absorbability may be formed from an aluminum alloy plate and the inner panel whose shape is complicated and has a large forming depth may be formed from a steel plate having excellent formability.
As a method of manufacturing a different materials panel structure by using the outer panel made of aluminum alloy and the inner panel made of steel, for example, a method of integrating the outer panel and the inner panel by hem processing (hemming or seam folding) in which peripheral edge portions of the outer panel are folded back and are caused to sandwich peripheral edge portions of flanges of the inner panel exists. In this method, ordinarily, an adhesive layer (resin layer) that is also an insulating layer for preventing electrolytic corrosion is formed between the outer panel and the inner panel by using a thermosetting resin, such as an epoxy resin, a polyester resin, or a phenol resin.
After the above-described different materials panel structure is assembled on an automobile body and subjected to coating, baking is performed at a high temperature in the range of 170 to 200° C. Here, due to displacement of a hem section caused by a difference between the linear expansion coefficient of the outer panel made of an aluminum alloy and the linear expansion coefficient of the inner panel made of steel, thermal deformation in which warping occurs at a side of the outer panel made of an aluminum alloy may occur. As a method of preventing such a thermal deformation, a method of, when integrating the inner panel and the outer panel with each other by hem processing, partly performing a TIG welding and joining operation on a predetermined location of each corner of the panel structure by using a flux cored wire (FCW) is proposed (refer to Patent Literature 1).
The method described in Patent Literature 1 above is very effective for preventing the hem section from being displaced if the welded joint can be provided with sufficient strength.
However, when joining different, materials, such as steel and aluminum, in other welding methods in addition to TIG welding, that is, MIG welding, laser welding, and spot welding, a fragile intermetallic compound (such as Al2Fe5, which is an aluminum-iron intermetallic compound) is formed in a joining interface of a welded portion, as a result of which a sufficient joining strength may not be obtained.
As a welding method that does not cause such an intermetallic compound to be formed, a method in which spot welding using a rivet made of the same type of material as one of the materials is performed is proposed (refer to Patent Literature 2).